A variety of electron microscopes and associated surface analyzers have evolved in recent years. One approach to chemometric surface analysis is electron spectroscopy for chemical analysis (ESCA) which involves irradiating a sample surface with ultraviolet or preferably x-rays and detecting the characteristic photoelectrons emitted. The latter method is also known as x-ray photoelectron spectroscopy (XPS). The photoelectrons are filtered by an electrostatic or magnetic analyzer which allow only electrons of a specified narrow energy band to pass through to a detector. The intensity of the detected beam typically represents the concentration of a given chemical constituent on or near a specimen surface. U.S. Pat. No. 3,766,381 (Watson) describes such a system.
Electron kinetic energies are detected or analyzed by magnetic or electrostatic devices that deflect charged particles according to their velocities. Electrostatic types include coaxial cylindrical, radial cylindrical and radial hemispherical. The Gerlach et al patent discloses the coaxial cylindrical analyzer. A radial cylindrical analyzer is taught in U.S. Pat. No. 4,764,673 (Bryson et al). The Watson patent and U.S. Pat. No. 4,358,680 (Read) describe the electrostatic hemispherical type of analyzer. Radial analyzers (cylindrical and hemispherical) generally are preceded by a lens system formed of rings, cylinders and/or grids for adjusting electron energy and focusing the electrons into the analyzer. Such lenses are disclosed in the aforementioned U.S. Pat. Nos. 4,358,680 and 4,764,673. The design of such lenses that vary retardation over a moderate range of retardation ratios is described in an article "Computer Optimization of Retarding Lens Systems for ESCA Spectrometers" by B. Wannberg and A. Skollermo, J. Electron Spectroscopy and Related Phenomena, 10 45-78 (1977). These systems are directed to area analyses using a relatively low solid angle of electron collection, and a wider range of ratios is desirable.
Another method for analyzing surfaces utilizes secondary Auger electrons generated at a small area of sample surface by a focused primary electron beam. Surface mapping of elements is accomplished by scanning with the primary electron beam. An example of a scanning Auger-microprobe utilizing a coaxial cylindrical type of electrostatic electron analyzer is provided in U.S. Pat. No. 4,048,498 (Gerlach et al).
A more commonly known instrument is a scanning electron microscope (SEM) in which a focused electron beam is rastered over a specimen surface. Secondary electrons emitted from the surface are detected in correlation with rastering positions. The secondary electron signals are processed electronically to provide a picture or image of topographical features of the surface. An SEM itself does not provide chemometric analysis, although x-ray emissions induced by the incident electrons are used for such analysis. Another limitation of the SEM is imaging the surface of electrical insulators, because of rapid charge buildup from the incident beam of electrons. Conductive coatings or other techniques are used to alleviate charging, but at the loss of surface details, time and cost of extra preparation, and loss of ability to remove surface layers during analysis. U.S. Pat. No. 5,118,941 (Larson), of the present assignee, discloses that insulator specimens can be imaged with a single frame of SEM rastering, but at the expense of resolution.
Separate detectors generally are used for the analyzing and imaging functions in the same instrument, for example as further disclosed in the Larson patent. This adds cost and employs space which could otherwise be available for other purposes. Also, particularly for low current systems, there is a need for detection of a greater proportion of electrons from the sample for imaging, and for rejecting background from stray electrons, ions and excited neutrals which originate from other sources. For energy analyzers the sample generally is kept field-free, whereas for imaging, a field is applied so as to maximize the quantity of electrons collected by the detector.
The Larson patent also discloses a system for locating target area for microanalysis of a specimen surface, using an SEM in conjunction with the microanalyzer. Backscattered electrons from the SEM electron beam are passed through the analyzer for producing a further image that is superimposed on the SEM image, such that the further image represents the target area for microanalysis.
Thus systems involving electron beam impingement on a specimen surface have evolved into high sensitivity instruments, in which very small areas may be selected for analysis. Rastering can be used to provide images or chemical mapping of the surface.
However, similar small-area sensitivity and raster mapping has been elusive for x-ray photoelectron spectroscopy (XPS).
X-rays from an anode target have been focused onto the specimen by means of a concave crystal monochrometer, as taught in U.S. Pat. Nos. 3,567,926 (Siegbahn), 3,617,741 (Siegbahn et al), 4,680,467 (Bryson et al), 4,752,685 (Shiokawa et al) and 5,127,028 (Wittry). A method of construction of a concave monochrometer for focusing x-rays is disclosed in U.S. Pat. No. 3,772,522 (Hammond et al), in which a quartz crystal disk is brazed with a metal film onto a concave spherical surface of a substrate. Because of a tendency of the disk to break during mounting to the curvature, a number of platelets may be bonded to the surface, for example in a monochrometer used in a PHI model 5600 instrument sold by Perkin-Elmer. Bonding techniques include brazing, optical contacting, epoxy and the like. The platelets are cut sequentially from the end of a single crystal rod of quartz.
A second approach to reduced area analysis has been to use an x-ray beam that floods the specimen surface, combined with a small-area objective lens for the photoelectrons, such as taught in U.S. Pat. No. Re. 33,275 (Wardell et al) for an electrostatic objective lens. Direct XPS imaging of a surface flooded with x-rays, using a type of magnetic lens variously known as an immersion lens, single pole piece lens or snorkel lens, as taught in U.S. Pat. Nos. 4,810,880 (Gerlach) and 4,810,879 (Walker).
Scanning for XPS may be effected by rastering the sample or the analyzer system mechanically, which is cumbersome. Scanning is also achieved by electronic deflection in the objective lens to receive electrons from off-axis, in a manner as described in an article "A Wide-angle Secondary Ion Probe for Organic Ion Imaging" by C. C. Grimm, R. T. Short, and P. J. Todd, J. Am Soc. Mass. Spectrum 1991, 2, 362-371. Scanning of the objective lens for generating photoelectron images is disclosed in the aforementioned U.S. patent No. 4,752,685, and in an article "AXIS: An Imaging X-Ray Photoelectron Spectrometer" by I. W. Drummond, F. J. Street, L. P. Ogden, and D. J. Surman, SCANNING 13, 149-163 (March-April 1991).
A higher resolution type of x-ray microscope utilizes zone plates and mirror techniques. This requires a very intense source of x-rays such as from a synchrotron, and so is not practical for general use.